1. Field of the Invention
This invention relates to rotary drive apparatus and, more particularly, to rotary drive apparatus having a driving member coupled by a cable to a driven member having a smooth outer peripheral surface.
2. Description of the Prior Art
U.S. Pat. No. 4,351,197 (Carson) discloses a rotary drive system in wich a threaded driving member is coupled to a driven member through a cable system. The driven member comprises a drum having a plurality of parallel grooves extending into the exterior surface. The coupling cable is disposed in the grooves on the exterior of the driven member and extends from each groove on the drum to adjacent threads on the driving member. The grooves on the driven member provide guidance for the cable as it moves onto and off of the driven member, and the grooves also provide additional surface for the frictional engagement of the cable. Thus, the cable is coupled frictionally to the driven drum in more than one location due to the presence of the grooves, if the grooves are V-shaped grooves, or there is frictional coupling over an extended area if the grooves are circular (semi-circular) to match the cable form.
The cost of machining the grooves on the driven member is an element to be considered in the overall cost of the rotary drive apparatus. In addition, the spacing between grooves on the drum must be correlated with the pitch of the threads on a driving member so that the cable goes onto and off of the drum and onto and off of the adjacent threads of the driving member at the same location. A small variation in the distance between the grooves on the driven member will, of course, result in potential problems as the driving member rotates to rotate the driven member.
To have a driven member without grooves is less expensive than the grooved member and, in addition, a greater degree of flexibility is allowed with respect to the diameter of the cable coupling the driving and driven members. Moreover, if the driven member is smooth, great flexibility results with respect to the number of consecutive turns about the driving member for every turn of the cable about the driven member. Thus, not only is the cost of the apparatus substantially reduced, but the inherent flexibility of the apparatus is substantially increased.